A hot rolling operation is carried out through a train of roughing rolls in which a plurality of vertical roll stands and a plurality of horizontal roll stands are arranged in series and alternately one after another. It has been the general practice in a rolling operation to set the roll gaps or the degrees of opening of the vertical roll stands at a desired roll opening prior to the feeding of the slab or material into the roll-stands, and such predetermined roll gaps are usually kept unchanged throughout the rolling operation for that material. Due to uneven distribution of temperatures or so-called skid marks in the slabs generated in the furnace, there is inevitably a possibility of variation in width of the rolled product such as strip and the like if such roll gaps are kept unchanged during the rolling operation. Because of this, in order to avoid the possibility of insufficient width in the rolled products, it has been the practice to intentionally add a substantial surplus width which would subsequently be trimmed to get a product having the desired dimensions. This procedure, however, results in substantial waste and reduction in yield.
In an attempt to eliminate or reduce variation in width of the rolled products due to such skid marks in the slab fed from the furnace, there has been proposed and disclosed the method for controlling the widths of materials to be rolled in the Japanese Patent Publication No. 26,503/1977 specification which comprises the steps of measuring temperature changes in a longitudinal direction of a material to be rolled upstream of the vertical roll stands, determining the deviation between a minimum temperature and a temperature at respective points of the rolled material, estimating variation in the material widths from the thus-obtained temperature deviation, determining a schedule for changing the roll gap or degree of opening of the vertical roll stands which is enough to compensate for the estimated variation in the material width, and controlling the roll openings of the vertical roll stands in accordance with the predetermined schedule for changing the roll opening during the rolling operation. This method is premised on the assumption that the cause of the width variation in a material is as follows.
That is, in a hot slab fed from the furnace, there would appear areas of a relatively high temperature and of a relatively low temperature alternately one after another in the longitudinal direction thereof under the effect of heat shielding due to the skid arrangement in the furnace. And, the relatively low temperature areas of the material or slab would have more resistance against the deformation than the relatively high temperature areas, with the result that a greater edging force is required in the relatively low temperature areas. On the other hand, since rigidity of the vertical rolls is relatively small, there would occur an effect of the large or small edging forces for these areas mentioned above, and therefore, it has been believed that the degree of width reduction by the vertical roll stands in the relatively low temperature areas would become smaller than that for the relatively high temperature areas, whereupon such variation might occur in the widths of the rolled bar or products.
The method of controlling the rolled product width according to the Japanese Patent Publication No. 26,503/1977 specification was on the assumption that such width variation would occur from the cause of uneven temperature distribution in the longitudinal direction of a slab to be rolled, and this led to adjusting the roll gap of the vertical rolls in accordance with the temperature variation existing in the longitudinal direction of the slabs or materials.
The well known prior methods of controlling the rolled strip widths along the entire longitudinal areas of the materials to be rolled are those disclosed in the Japanese Patent Publication No. 25,823/1976, the Japanese Patent Laid-Open Application No. 72,350/1977, the Japanese Patent Publication No. 24,907/1975, and the Japanese Patent Laid-Open Application No. 87,455/1976. In the method disclosed in the Japanese Publication No. 25,823/1976, a slab or material is rolled by the vertical rolls in such a manner that the width spread in a material rolled therefrom will merely be relatively increased or decreased in inverse proportion to the width of a material to be rolled.
The method disclosed in the Japanese Patent Laid-Open Application No. 72,350/1977 is directed to the control of roll gap of the vertical rolls and is based on the assumption that the amount of width spread in a material obtained by the vertical rolling operation followed by the horizontal rolling operation is determined as a function of the longitudinal position (distance from the leading end) of the material, which comprises the steps of applying the thus-presumed relationship as a width spread factor to each combination of the thickness and the width of the material, and setting a pattern to be utilized for controlling the opening of the vertical rolls so as to control the above mentioned width spread according to the factor above. The method disclosed in the Japanese Patent Publication No. 24,907/1975 is concerned with the control of the roll gap of the vertical rolls and is premised on the calculation based on the equation relative to the width spread of the material to be rolled which is determined solely in connection with the given rolling conditions, while the method disclosed in the Japanese Patent Laid-Open Application No. 87,455/1976 is also concerned with the provision of the gap control of the vertical rolls in consideration of the state of width spread of the material to be rolled which is likewise defined exclusively under the rolling conditions.